There is no good reason at for any kind of ethic burden.
Some people steal; some might have even stole something from you. Doesn't mean you have to too, and I doubt most people are held back only by law/punishment.
I don't think you can equate me leaving an employer "early", for some definition of early, to theft. As long as the contract is "at will employment", which majority of American job offers are (sorry for my America centric view), then the contract is AT WILL, in both directions. In my mind there is no ethical, moral or legal lapse.